The current mainstream in the semiconductor industry resides in diodes, transistors, ICs, LSIs and VLSIs of the resin encapsulation type. Epoxy resins have superior moldability, adhesion, electrical properties, mechanical properties, and moisture resistance to other thermosetting resins. It is thus a common practice to encapsulate semiconductor devices with epoxy resin compositions. Semiconductor devices are now used in every area of the modern society, for example, in electric appliances and computers. As a guard against accidental fire, the semiconductor encapsulating materials are required to be flame retardant.
Halogenated epoxy resins combined with antimony trioxide are often blended in epoxy resin compositions in order to enhance flame retardance. This combination of a halogenated epoxy resin with antimony trioxide has great radical-trapping and air-shielding effects in the vapor phase, thus conferring a high fire-retarding effect. However, halogenated epoxy resins generate noxious gases during combustion, and antimony trioxide has powder toxicity. Given their negative impact on human health and the environment, it is desirable to entirely exclude these fire retardants from resin compositions.
In view of the above demand, studies have been conducted on the use of hydroxides such as Al(OH).sub.3 and Mg(OH).sub.2 or phosphorus-containing fire retardants in place of halogenated epoxy resins and antimony trioxide. Unfortunately, because of various problems associated with the use of these alternative compounds, such as inferior curability of the resin composition during molding and poor moisture resistance in the cured product, they are not yet ready for practical application.
Of the phosphorus-containing fire retardants, red phosphorus is effective for imparting a very high flame retardance to the composition since it forms a phosphoric acid compound during combustion, which bonds with a char layer to form a flame retardant film. However, red phosphorus needs careful handling because of inferior compound stability and an ignition danger upon impact. Additionally, red phosphorus has a serious drawback that if it is added to semiconductor encapsulating epoxy resin compositions, it gradually forms a phosphoric acid compound which can corrode aluminum wiring in semiconductor devices, resulting in a loss of reliability. Red phosphorus is thus inapplicable to semiconductor encapsulating epoxy resin compositions.